The invention relates to an optical recording medium having a substrate and a stack of thin layers provided thereon, the stack comprising a first dielectric layer and a second dielectric layer, a recording layer being able to change between two crystallographic states and arranged between the dielectric layers, a third dielectric layer arranged between the recording layer and the first dielectric layer, and a reflective layer.
The invention also relates to the use of such an optical recording medium in high storage density and high data rate applications.
Optical information or data storage based on the phase change principle is attractive, because it combines the possibilities of direct overwrite (DOW) and high storage density with easy compatibility with read-only systems. Phase-change optical recording involves the formation of micrometer-sized and sub-micrometer-sized amorphous recording marks in a thin crystalline film using a focused radiation beam. During recording information, the medium is moved with respect to the focused radiation beam which is modulated in accordance with the information to be recorded. After heating by the radiation beam to a temperature above the melting temperature, the recording layer is quenched, thereby causing the formation of amorphous information marks in the exposed areas of the recording layer which remains crystalline in the unexposed areas. Erasure of written amorphous marks is realized by heating the recording layer with a radiation beam to a temperature below the melting temperature during a time longer than the crystallization time of the recording layer, allowing the recording layer at the position of the mark to recrystallize. The amorphous marks represent the data bits, which can be reproduced by scanning the recording medium through the substrate by a low-power focused radiation beam. Reflection differences of the amorphous marks with respect to the crystalline recording layer bring about a modulated laser-light beam which is subsequently converted by a detector into a modulated photocurrent in accordance with the coded, recorded digital information. The modulated photocurrent is an HF information signal representing the written marks, and, therefore, the written information.
The main problems in phase-change optical recording are the required large number of overwrite cycles (cyclability), i.e. the number of repeated writing (amorphization) and erasing (recrystallization) operations, and a proper crystallization speed. High crystallization speed is particularly required in high data rate applications, where the complete crystallization time has to be shorter than 100 ns, preferably as short as 30 to 70 ns. If the crystallization speed is not high enough to match the linear velocity of the medium relative to the laser-light beam, the old data (amorphous marks) from the previous recording will not be completely erased (recrystallized) during direct overwrite (DOW). This will cause a high noise level.
An optical recording medium of the type mentioned in the opening paragraph is known from U.S. Pat. No. 5,234,737. The known medium of the phase-change type comprises a disc-shaped substrate carrying a stack of layers consisting, in succession, of a first dielectric layer, a recording layer of a phase-change Ge-Sb-Te alloy, a second dielectric layer and a metal reflective layer. Such a stack of layers can be referred to as an IPIM structure, wherein M represents a reflective or mirror layer, I represents a dielectric layer and P represents a phase-change recording layer. A third dielectric layer, preferably of SiO.sub.2, has been arranged between the first dielectric layer and the recording layer to achieve a desired thermal behaviour of the medium during writing information in the medium by means of a radiation beam. The material of the third dielectric layer may also be chosen from MgF.sub.2, CaF.sub.2, Al.sub.2 O.sub.3, CeF.sub.3, NdF.sub.3, LaF.sub.3, NaF, NiF, Na.sub.3 LaF.sub.6, ZnS, ZnS-SiO.sub.2, SiC, In.sub.2 O.sub.3, SnO.sub.2, Si.sub.3 N.sub.4, Ta.sub.2 O.sub.5, SiO, TiO, TiO.sub.2, TiN, TiC and ZrO.sub.2.
The above citations are hereby incorporated in whole by reference.